In modern motor vehicles, the fuel injection systems used make a major contribution to fulfilling demanding customer and legal requirements in respect of fuel consumption and emissions of unwanted pollutants. Modern motor vehicles of this kind have self-ignition internal combustion engines which operate with a common rail diesel injection system, for example.
Faults which occur in such systems, e.g. leaks, mechanical component failure, contamination etc., often lead to unwanted vehicle behavior, e.g. a loss of power, increased pollutant emissions or activation of a fault memory lamp. Faults of this kind can occur or have their origin in the low pressure area of the respective vehicle or in the high pressure area of the respective vehicle.
Known onboard diagnostic systems have only a limited ability to determine the exact cause of a fault in the injection system or at least localize it more precisely without having a negative effect on the behavior of the overall injection system during diagnosis, especially when dynamic operating conditions are present. In addition, precise location of a cause of a fault is considerably restricted by the fact that only a limited number of items of onboard sensor information are available.
One consequence of the abovementioned problems is that components are often replaced unnecessarily in a workshop for lack of a precise knowledge of the cause of a fault. For example, a functional high pressure pump may be replaced even though the unwanted system behavior has been caused by a blocked fuel filter.
Moreover, the practice of attaching additional sensors to the fuel injection system and carrying out manual tests for diagnostic purposes in a workshop is already known. However, this is associated with a large outlay on analytical equipment for the respective workshop, this in turn increasing the readiness to unnecessarily replace components which are actually functional. In addition, manual interventions in the high pressure system of a motor vehicle often lead to contaminants being introduced into the system or to components of the system being damaged.
DE 197 27 794 C1 has already disclosed a method for checking a fuel supply system in a motor vehicle, said system delivering fuel from a fuel pump to an injection system of an internal combustion engine. In this known method, a change in the fuel pressure in the fuel line over time after the internal combustion engine is switched off by switching off the fuel pump and injection system is monitored for a predetermined period of time . The change in the fuel pressure is compared with a comparison characteristic, which depends on the temperature of the fuel. If there is a deviation of more than a predetermined tolerance range, a malfunction is detected. By means of this known method, malfunctions in the high pressure area of the injection system are detected. However, there is no possibility of making judgments on faults in the low pressure area.
DE 196 22 757 B4 has disclosed a method and a device for detecting a leak in a fuel supply system of an internal combustion engine having high pressure injection. Here, the fuel is delivered from a low pressure area to a high pressure area by at least one pump. The pressure in the high pressure area can be controlled by at least one pressure control means. To detect the pressure in the high pressure area, a pressure sensor is provided. When the internal combustion engine is started, at least one of the pressure control means can be activated in such a way that, in the fault-free state, the pressure rises to an expected value. The presence of a fault is inferred if the pressure value detected does not reach the expected pressure value within a predetermined period of time. This known method makes it possible to detect a fault in the injection system. It is not possible to differentiate between the high pressure side and the low pressure side. Moreover, the potential for detection is limited since, after the internal combustion engine is started, the low starter speed which is then present leads to only a relatively low flow through the pump. The result is that the causes of faults which have an effect on the behavior of the vehicle only at higher flow rates, e.g. a blocked fuel filter, cannot be detected. Moreover, the maximum permissible fuel pressure is severely limited in the presence of the comparatively low starter speed in order to ensure adequate pump lubrication despite the low flow through the pump. This has the effect that it is not possible to evaluate the entire pressure range by means of the known method, and hence the number of detectable causes of faults is limited.